Method of making high strength rotating elements



March 3, 1953 R. B. JOHNSON, JR

Filed Aug. 25, 1949 DIRECTION OF F155)? FLOW 0R JTRUCTURE.

DIRECTION OF FIBER FLOW 0! STRUCTURE.

METHOD OF MAKING HIGH STRENGTH ROTATING ELEMENTS m m 5 mm Wm. i n 25 9M.hi 0 P W His Attorney.

Patented Mar. 3, 1953 METHOD OF MAKING men STRENGTH ROTATING ELEMENTSRobert B. Johnson, .lr., Melrose, Mass, assignor to General ElectricCompany, a corporation of New York Application August 23, 1949, SerialNo. 111,858

1 Claim. (01. 29-156-8) The present invention relates to high strengthrotating elements and to the method of making,

the same. More particularly, my invention relates to the production ofhigh strength, highspeed wheels, e. g., turbine wheels, such as thoseemployed in turbosuperchargers, adapted to op erate at speeds of atleast 20,000 R. M. P., and in some cases as high as 60,000 R. P. M.

A primary problem connected with high-speed turbine wheels has beenpremature bursting at high speeds. Various alloys have been developedwith the object of providing a material meeting the physicalrequirements for such wheels. However, up to the present time, suchattempts to solve the problem have not been entirely successful.

The present invention is based on the discovery that the defect in thepresent forged wheels re-.- sults from a low ductility of the centersection thereof as compared with the remaining portions of the wheel.This difference in ductility has in turn been found to be inherent inthe method now employed in making the wheels and practically independentof the alloy employed. It is, therefore, a primary object of the presentinvention to provide improved high strength wheels. A further object isto provide an improved method of forging high-speed wheels, particularlyturbosupercharger wheels, whereby forged wheels can be obtained havinggreater ductility and higher strengths particularly at and adjacent tothe center section of the forgings.

Further objects and features of the invention will become apparent inthe following description in connection with the accompanying drawinginwhich Figs. l-3, inclusive, represent the various steps presentlyfollowed in the manufacture of forged wheels and Figs. 4-6, inclusive,represent the essential steps in the practice of the present invention.

The forging method of the present invention will best be understood byfirst considering the prior practice in the manufacture of forgedsupercharger wheels. With reference to the drawing, this practice, forexample, makes use of a forging slug I cut a indicated from a 4-inchsquare, hot-rolled billet 2 in which the fiber s t 1 '1 1 cture islongitudinal of the billet. These lugs, about 13 inches in length, areupset along their long axis, i. e., parallel to the fiber structure,using fiat upset dies, to a height of about 6 /2 inches, as indicated inFig. 2. In the drawing the various arrows indicate the forging directionwhile the grain or fiber flow lines resulting from the rolling operationare indicated on each of the figures. The upsetting operation i carriedout at an elevated temperature of, for example, 1950 F. Thereafter, theupset slugs are forged in standard wheel dies, for example, by first hotforging the upset slug at a temperature of 1950 F. to the point wherethe rim of the wheel is about 1% inches thick, solution treating theforging at 2150 F., followed by air cooling, and finally forging thewheel blank to a final 1 inch rim thickness at a temperature of about1200 F. The forged Wheel (Fig. 3) is then given a strain-relief annealat l200 F. for 8 hours and air cooled before being machined andotherwise finished.

Considering thi process, it will be noted that due to the methodemployed in hot rolling the billets from the original ingot, thereexists in the center section of the billet, regardless of itscrosssectional area, a longitudinally extending portion 3 ofsubstantially unworked material. Not only has this center portion orso-called dead spot received the least amount of working during therolling operations but, in addition, it also contains the usualsegregations and inclusions normally present in the center of the ingot.Further, in following the present procedure of forging wheel blanks fromslugs cut from such billets, the dead spot material receives the leastamount of working during the forging of the slug into the form of thefinished wheel. To the further detriment of the forged product, thesubstantially unworked center portion 3 of the billet comprises thecenter or axial portion 4 of the forged product where the physicalproperties are most critical. As an example of the results obtained bythe present forging practice, test bars cut from the center section ofsuch forgings frequently exhibit yield strengths as low as 60,000 p. s.i. and elonagtion values less than 3 per cent. The alloy employed inthese wheels was a high strength alloy of about 16 per cent chromium, 25per cent nickel, 6 per cent molybdenum, 0.7 per cent siliccn, 1.35 percent manganese, 0.1 per cent carbon, balance iron. In the overspeedtesting before attaching buckets, such wheel forgings ordinarily burstat about 48,000 to 50,000 R. P. M.

The present invention provides a method whereby stronger and moreductile Wheels can be obtained and by which it is assured that completeworking of the dead spot center material of the original hot-rolledbillet is attained with the resultant elimination of this dead spotmaterial from the center portion of the forged product.

The hot-rolled billet from which the slugs are cut is larger than thoseheretofore employed and is of such a cross-sectional area that aplurality of slugs can be cut from a single billet across section. Eachof the slugs so obtained will contain a portion of the substantiallyunworked center material of the billet along a surface portion of theforging slug, such as, for example, a longitudinal edge of the forgingslug, thus permitting such portion of the substantially un-. workedcenter material to be worked upon in subsequent operations. The mannerin which these slugs can be obtained will become more apparent from theconsideration of Fig. 4 illustrating a hot-rolled billet substantially'four "times the cross-section area of the hot-rolled billet of Fig. 1and having a substantially unworked center portion 6. Cutting alongdotted line 1, there is obtained a billet section of the proper lengthas, for example, the same length as slug I. This section is thenquartered by two longitudinal cuts along lines 8 and 9 to obtain fourforging slugs with the substantially unworked center portion of theoriginal billet divided equally among the slugs with a portion thereofextending along only one longitudinal edge of each slug.

These slugs are then upset and die forged in accordance with the usualpractice outlined hereinbefore. However, during the various steps, the

"dead spot material is forged and substantially worked, particularlyduring the time the upset slug l illustrated in Fig. is converted to theforged wheel ll of Fig. 6. This working refines and breaks up anysegregation that might have occurred during the freezing of the ingotfrom which the hot-rolled billet was obtained. In fact, I

as will be evident from the consideration of the shape of the finalforging of Fig. 6, the longitudinal edge portions of the forging slugswhich constitute part of the material from which the rim of the wheelblank is formed are among those portions of the slug which receive themaximum working during the forging of the slug. Hence, since the "deadspot material 6 comprises one edge of each of the slugs employed in thepractice of the present invention,'substantial working of this criticalsection of the slug is obtained. In addition, the critical centerportion I 2 of the forged wheel is composed of material which is free ofthe ingot segregations and which also received a substantial amount ofworking during the rolling of the billet. As a result, better physicalproperties can consistently be obtainedin the center portion.

A further and important advantage of the present invention is the factthat the quartering operation permits the inspection of the center ofthe original billet for major defects which under the former practicewould unknowingly be forged into the center of the wheel.

Wheel forgings prepared in accordance with the present or quarteredbillet practice are definitely superior to those made by the usual orconventional method as is shown by the test results set forth in thefollowing table. The values given are average values for the'same sizewheel forgings which with buckets attached have an overall diameter of9.5 inches and are designed to operate at 30,000 R. P. M. f In all casesthe alloy hereinbefore described was employed.

Conven- Quartered tional Billet Method Method Yield Strength (02%ofiset) ......p. s. i 70,008 73,038 Tensile Strength p. S. 1.. 108,902118, 38 Elongation .percent 11.65 20. 35 Reduction .do.... 14. 50 2 .17

, forgings prior to bucketing have shown the present wheel forgings canbe run as high as 60,000

'RZ'P. M. without bursting, whereas wheel forgings. of this type andsize forged by the conventional method ordinarily burst at speed of from48,000 to 50,000 R. P. M. As a result of the present invention, thepresent physical requirements of turbosupercharger wheels can readily bemet and failures or rejects substantially eliminated. While theinvention has been described and illustrated with reference specificallyto a turbine wheel forged from a particular alloy, it is apparent thatit is applicable to the production of any rotating element, composed ofany suitable alloy, where high bursting strength during operation is animportant consideration. Also, it is obvious that while the method ofattaining the improved high strength wheel has been illustrated by thequartering of the billet, the invention is notlimited thereto since theobjects sought may be attained by any subdivision of the billet wherebythe "dead center" portion is presented at or adjacent to an exposedsurface portion of the slug or slugs cut from the billet.

What I claim as new and desired to secure by Letters Patent of theUnited States is:

The method of making a high speed turbine wheel which comprisesquartering a hot-rolled billet in the rolling direction to obtain aforging slug in which a longitudinal edge of the slug extending parallelto said rolling direction is composed of part of the substantiallyunworked center portion of the hot-rolled billet and the center of theslug is composed of material intermediate the center and surfaceportions of said billet, upset forging said slug in a direction parallelto the rolling direction of said billet and die forging the upset sluginto the form of a wheel blank having a rim portion of a thickness lessthan the thickness of its hub portion by the application of forgingpressure in the same direction as that applied in the upset forging ofthe slug whereby the material constituting the rim portion of the wheelblank and including the material forming part of the center portion ofthe hot-rolled billet receives substantial working during die forgingstep.

ROBERT B. JOHNSON, JR.

7 REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 878,871 Dods Feb. 11, 1908942,489 Dods Dec. 7, 1909 1,397,566 Walter Nov. 22, 1921 1,410,093Dallmeyer Mar. 21, 1922 1,486,365 Cummings Mar. 11, 1924 1,792,581 FirthFeb. 17, 1931 FOREIGN PATENTS Number Country Date 799,408 7 France Apr.4, 1936

